Fluidized bed catalytic cracking (commonly referred to as FCC) processes were developed during the 1940's to increase the quantity of naphtha boiling range hydrocarbons which could be obtained from crude oil. Fluidized catalytic cracking processes are now in widespread commercial use in petroleum refineries to produce lighter boiling point hydrocarbons from heavier feedstocks such as atmospheric reduced crudes or vacuum gas oils. Such processes are utilized to reduce the average molecular weight of various petroleum-derived feed streams and thereby produce lighter products, which have a higher monetary value than heavy fractions. Though the feed to an FCC process is usually a petroleum-derived material, liquids derived from tar sands, oil shale or coal liquefaction may be charged to an FCC process. Today, FCC processes are also used for the cracking of heavy oil and reduced crudes. Although these processes are often used as reduced crude conversion, use of the term FCC in this description applies to heavy oil cracking processes as well.
The operation of the FCC process is well known to those acquainted with processes for upgrading hydrocarbon feedstocks. Differing designs of FCC units may be seen in the articles at page 102 of the May 15, 1972 edition and at page 65 of the Oct. 8, 1973 edition of "The Oil & Gas Journal". Other examples of FCC processes can be found in U.S. Pat. Nos. 4,364,905 (Fahrig et al); 4,051,013 (Strother); 3,894,932 (Owen); and 4,419,221 (Castagnos, Jr. et al) and the other FCC patent references discussed herein.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores and upon the external surface of the catalyst. Although the quantity of hydrocarbon retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. It is important to remove retained spent hydrocarbons from the spent catalyst for process and economic reasons. First, hydrocarbons that enter the regenerator increase its carbon-burning load and can result in excessive regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products. The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, countercurrent to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and hydrocarbons which are adsorbed on the catalyst.
The efficiency of catalyst stripping has been increased by using a series of baffles in a stripping apparatus to cascade the catalyst from side to side as it moves down the stripping apparatus. Moving the catalyst horizontally increases contact between it and the stripping medium. Increasing the contact between the stripping medium and catalyst removes more hydrocarbons from the catalyst. As shown by U.S. Pat. No. 2,440,625, the use of angled guides for increasing contact between the stripping medium and catalyst has been known since 1944. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of similar stripping devices for FCC units are shown in U.S. Pat. Nos. 2,440,620; 2,612,438; 3,894,932; 4,414,100; and 4,364,905. These references show the typical stripper arrangement having a stripper vessel, a series of baffles in the form of frusto-conical sections that direct the catalyst inward onto a baffle in a series of centrally located conical or frusto conical baffles that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffle in the series and continues rising upward from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 and the use of multiple linear baffle sections at different baffle levels as demonstrated by FIG. 3 of U.S. Pat. No. 4,500,423. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 where a quantity of fluidizing gas is admitted at a number of discrete locations.
As previously mentioned for reasons of heat balance and product recovery, improvements in the efficiency of FCC stripping are particularly desirable. One way to improve FCC stripping efficiency is to increase the contact time between the stripping fluid and the FCC catalyst. This can be done by extending the length of the FCC stripping zone. The extended length increases efficiency by increasing the relative partial pressure of the stripping fluid, typically steam, in the lower portion of the stripper from which the catalyst normally exits. Furthermore, higher additions of stripping fluid such as steam can also raise the steam partial pressure within the stripping zone thereby serving to further reduce the carryover of hydrocarbons from the stripping zone into the regenerator. However, increasing the length of the stripping zone, or adding additional stripping steam to the FCC stripper, increases the cost of the operation of the unit as well as burdening downstream recovery facilities by the extra circulation and recovery of water. As a result, methods are sought to improve the recovery of hydrocarbons from FCC catalyst without increasing the length, or adding additional quantities of steam to the stripping zone.